1. Field of the Invention
The present invention relates to a telecommunication optical fiber cable and in particular it relates to a reduced diameter dielectric optical fiber cable with improved installation features for use in the end part of an access telecommunication network.
2. Description of the Related Art
Access telecommunication networks made by copper wires are being replaced by optical fiber networks in view of their large bandwidth capabilities. As the replacement is submitted to the effective final client requests and is rather expensive for a telecommunication provider, some providers use to arrange an access network made only of empty plastic conduits and to lay the optical fiber cables in the conduits only when proper requests from the clients have been received. Such access network optical cables could comprise a reduced number of optical fibers, typically 2÷12, 24, 48 or 72.
A suitable technique to install these optical cables in the conduits is the “blown method”: the optical fibre cable is propelled along a previously installed duct by fluid drag of a gaseous medium, preferably air, blown along the duct in the desired direction of a cable advance. Blowing methods are deemed to be profitable for installing cables in long and short routes due to the lower cost, short time and low tension on the cable.
Advantageously, an optical cable which is able to be profitably installed in the above mentioned conduits by blowing methods should have a rather reduced diameter and a rather reduced weight. Such requirements are common to all the cables to be blowing installed but are much more important for those that are designed for the end access part of the network, that is characterized by a high number of direction changes (tortuous path).
The two well-known structures of optical cable are the multi-loose tube (MLT), wherein a plurality of tubes loosely house the optical fibers and are arranged around a central strength member, and the central-loose tube (CLT), wherein the fibers are loosely housed in a single central tube and the required strength of the cable is provided by other means, for example by two lateral rods.
MLT dielectric optical fiber cables comprising up to 24 optical fibers and fit for blowing installation in existing conduits are known in the art. Unfortunately, such multi loose tube optical cables, by their nature, are not easily miniaturized. For instance, MLT optical cables are known which comprise four tubes (with six optical fibers for each of them), with each tube having an outer diameter of 2.2 mm and an inner diameter of 1.5 mm, and having an outer diameter of about 6.3 mm. Other MLT optical cables are known which comprise six tubes (with four optical fibers for each of them), with each tube having an outer diameter of 1.9 mm and an inner diameter of 1.2 mm, and having an outer diameter of about 6.7 mm. The relatively large dimensions of such cable imposes the use of conduits of at least 10 mm of outer diameter.
CLT optical fiber cables suitable for blown installation are described, for example, in the article of W. Griffioen et al., “Versatile Optical Access Network for Business and Future Consumer Market”, Communication Cables and Related Technologies, A. L. Harmer (Ed.) IOS Press., 1999, pp. 69-75. Such cables comprise a steel welded tube which is covered by a HDPE (High Density Polyethylene) based mixture. These cables may have an outer diameter of about 4 mm (those comprising 2÷12 fibers) or about 6 mm (those comprising 24÷48 fibers) so that they require to be installed in conduits (typically made of plastic) having inner and outer diameters of 7 mm and 10 mm, respectively. It is also observed that the above CLT cables are not dielectric and such a requirement is generally a key one for the local access networks that are high sensitive to electromagnetic fields and phenomena.
In view of the above, there is the need of an optical cable comprising a rather low number of optical fibers, typically up to 24 optical fibers, which is dielectric, which is usable in a wide range of temperatures, (typically from about −30° C. to about +60° C., for outdoor applications, and from about −10° C. to about +60° C. for indoor applications) which is installable by blowing techniques in a tube of relative reduced dimensions (typically having outer diameter of about 7 mm and an inner diameter of about 5.5 mm) and finally which has a rather reduced diameter, typically about 4.0÷4.5 mm.
The Applicant has considered that a CLT cable structure is particularly suitable to achieve these goals. In other words, a profitable optical cable structure providing dielectric, temperature resistance and reduced size features is the one comprising: a core tube containing optical fibers, a plastic jacket that surrounds the core tube, and a pair of linearly extending, diametrically opposed dielectric rods that are at least partially embedded in the jacket, with the rods having a compressive stiffness that is effective to inhibit substantial contraction of the cable and a tensile stiffness that is effective to receive a tensile load without substantial transfer of the tensile load to the optical fibers. The rods provide both tensile and compressive modulus and strength so that their compressive properties are sufficient to inhibit shrinkage of the plastic jacket and to resist buckling during handling of the cable.
The above described type of cable with the core tube and a pair of dielectric reinforcing rods simmetrically positioned on opposite sides of the core tube is characterized by an asymmetric bending behavior. In particular, a cable of the above type exhibits a bending stiffness in the plane containing the two reinforcing rods that is higher than the bending stiffness in the plane which is orthogonal to the plane passing through the reinforcing rods. In other words, a similar cable exhibits preferential bending plane. It is known to the man skilled in the art that the cables based on such a structure may have reduced performances in terms of maximum cable lengths that can be introduced in a tube, the installation performances being even lower when blowing techniques are used.
US 2003/0044139, which is considered the closest prior art, describes a known CLT optical cable which is reinforced by a pair of diametrically opposed strength rods. According to US 2003/0044139, the bending resistance between orthogonal bending planes can differ by a factor of only 1.2 if the rods are surrounded by a frictional adhesion coating that enables them to move locally within the jacket in response to compressive or flexural stress applied to the cable, while without that coating the same factor would be four. According to the teachings of US 2003/0044139, this increases the blowing performances of the cable. The cable described in US 2003/0044139 is a cable containing a relatively large number of optical fibers and has a rather large diameter reinforcing rods (1.5÷3.0 mm). It is therefore likely that said cable has a relatively high diameter and is designed for application in the backbone of a telecommunication network, which is not so tortuous as the path in the final/access part of the network itself.